In connection with the third, it may be said that it is not a frequent

cause of failure, as provision is almost always made to prevent it, but

it may occur in a reservoir embankment from extraordinary circumstances,

or in a much less degree from sufficiently high waves being generated

upon a lake or impounding reservoir that the top of the bank may be

loosened, and the water dash over it and erode the land slope, and

convert it into a kind of tail-race; tarpaulins have been temporarily

laid upon the surface on an emergency to protect a soft place, also sand

bags and planking to prevent an overflow. In an embankment erected for

such a purpose the top width is more severely strained, consequent upon

the greater exposure, than in a reservoir embankment, and the height

must be sufficient to prevent water passing over it.

With reference to the fourth cause of failure, “boils” in loose soil and

the bursting of springs are referred to in Chapter XII.

When a reservoir is emptied the weight of the water on its bottom is

removed, but the load from the embankment is the same, and should the

ground be soft the embankment may subside towards the reservoir and the

bed be uplifted; hence it may be advisable not to draw off the water

unless the bottom is weighted or movement prevented.

Due regard of the different causes of failure herein enumerated and care

in construction, will reduce to a minimum the probability of a slip or a

subsidence in an earth embankment erected to contain or expel water.

The temporary or permanent diversion of rivers or streams being so often

necessary in public works, a few paragraphs are here devoted to it so

far as regards earthslips and subsidences. If it be possible when the

soil is very porous and incapable of retaining water, it is advisable

not to divert a river.

Some of the most vulnerable places in a newly-formed river-bank are:

The ends that join it to the old bank or to the land, to which it should

be thoroughly connected.

The toe of the slope and seat, which should be tied into the bed of the

river or be well protected by making the slope flat towards the base.

Any abrupt bends or angles should always be avoided as they increase

erosive action.

The wind and water line which requires especial protection.

Provided these points are remembered and the usual precautions taken in

forming a river-bank to make it thoroughly sound and homogeneous, a slip

or subsidence of serious moment is improbable.

In order to protect the sandy bed of a river and to prevent the banks

slipping and subsiding, it may be necessary to guard against scour of

the bed and consequently of the toe of the banks. Stone thrown in will

settle and compress the bed by weighting and consolidation. By

periodical depositions the sand becomes more protected and the quantity

of stone required is reduced, but especial care should be taken to

preserve the normal bed, to offer no obstruction, and not to cause

whirlpools or to interfere with the current except to direct and train

it, or the erosive action so created will cause movement. Stones simply

cast in and allowed to sink and find a permanent bed until the regular

surface of the bottom of a river is so reached, have been proved in many

instances to be a sure protection in sandy soils provided eddies do not

exist. The preservation of the slopes is particularly referred to in

Chapter VII.

CHAPTER XI.

NOTES UPON THE FAILURE OF DOCK AND OTHER WALLS FROM A FORWARD MOVEMENT

OF THE EARTH FILLING OR BACKING.—CONSIDERATION OF THE CAUSES OF SUCH

ACTION AND SOME PREVENTIVE AND REMEDIAL MEASURES.

The most serious consequences often result from slips or “boils” of sand

or loose soil during the construction of docks, and from the sliding

forward or subsidence of quay walls, as in addition to the great expense

of restoring the damaged wall must be considered the loss of receipts

caused either by delay in opening the docks or from interference with

the traffic. In this chapter no attempt is made to lay down rules and to

declare that if they are followed failures from pressure, subsidence, or

a slip of earth, can be obviated by careful attention to them. Many of

the most renowned engineers and contractors throughout the world, whose

scientific attainments and practical knowledge are universally and

deferentially acknowledged by those competent to form a judgment, have

had anything but pleasant experiences in the construction of docks and

similar works.

The two banes of dockworks may be said to be:—

1. Defective foundations.

2. “Boils,” or the rushing up of loose soil and water in the

foundations.

To say that a failure of a dock-wall cannot but be the result of

defective engineering is a statement which it would be difficult to

substantiate. To disturb the earth below the required depth of the

foundations at intervals over the area of such works in treacherous

soils in order to ascertain its character would simply be to offer a

general invitation to it to commence movement, _vide_ p. 208. It would

be worthy to rank with the erudite report of a local committee formed to

declare the best means of preventing floods in a district, which, after

deep consideration reported: “That the committee were of opinion the

floods would be permanently removed provided the engineer diverts all

rivers, streams, watercourses, and ditches, to the next valley, and

closes the up-stream hole in the hill, or causes the water to be

discharged elsewhere, and reduces the rainfall within the limits

required from time to time by the inhabitants of the district.”

Inasmuch as it is impracticable to ascertain the exact character and

condition of any earth over a large area in such a location as that

usually occupied by a dock, and to be certain that no unstable seams

exist, and as the forces of nature are infinitely more powerful than

those of man, it cannot be said that a failure is absolutely impossible.

All that can be done is to render improbable any slip or subsidence by

the employment of every known means to promote stability justified by

the importance of the work and with a due regard to economy.

Dock-works are peculiarly liable to slips and subsidences because of

their position necessarily being near to the sea, an estuary, river, or

lake, and therefore in usually treacherous superficial soil of an

insecure, water-charged character. The foundations are also at but

comparatively little depth below the lowest water-level, and although

they are not subject to the frequent vibration railway cuttings and

embankments have to withstand, they are jarred by blows from vessels and

falls of heavy goods, vibration from cranes and machinery, and also are

liable to be suddenly and unevenly surcharged. The earth backing may

also become quickly water-charged, and a force caused by the tide

varying in height has to be considered in earth embankments for docks or

reclamation purposes, particularly in sandy or loose soil, namely, that

of the foundations being “blown” up by the upward pressure of the tidal

or permanent head of water above the level of the bottom of the

foundation; thus inside an enclosure embankment, or cofferdam, a depth

of water of 30 feet would give an upward pressure of, say, 0·85 ton per

square foot, and when the excavation has extended to a depth of 30 feet

an insistent natural weight of a column of sand 1 foot square upon the

foundation has been removed by the excavation equal to 1·65 ton, _vide_

Chapter V. Thus the upward and relieved pressure 0·85 + 1·65 equals 2·50

tons per square foot, 0·85 ton of which constantly varies. It is obvious

that in very permeable soils, such as sand, the hydrostatic pressure

will be most felt, and should the range of tide be considerable the

strain upon the earth will change and an active disturbing force be

constantly present. This varying disturbing agent must be seriously

considered in all dockwork or excavation near the sea or a tidal river,

and also approximate to water, for there will always be danger of the

water percolating, and when the foundations are not sufficiently deep of

their being undermined. The weight and depth must therefore be

sufficient to balance or exceed any upward pressure of the water in the

soil due to its head level. The bottom of the foundation will almost

invariably be below the depth at which water is usually found in the

locality, and it should not be forgotten that sooner or later the earth

upon which dock-walls rest is nearly certain to become wet and perhaps

saturated, and that when saturated any additional water must flow away.

In sandy or loose soils this may become dangerous. Care must also be

taken in loose soil that it is not loaded by any temporary works so as

to force the earth in any direction, or a slip will occur and any

trenches may become filled, and every effort should be made to cause

equal loading and pressure and to prevent unequal settlement.

The selection of the site of a dock is generally confined within narrow

limits such as railway connection, roads, location of the want of dock

accommodation, currents, the proper position of the entrances, &c., &c.;

nevertheless the nature of the soil should be considered, for upon a

tidal river a few hundred feet one way or other may cause the works to

be upon various kinds of earth each more or less treacherous, and when

intermixed perhaps more unstable. Even upon one side of a comparatively

narrow river the earth may be in a much more stable condition than upon

the other; and when an old watercourse or a dry ditch passes across the

site of a dock it must not be disregarded, as failures and damage have

occurred because the same section of a wall was continued throughout. It

is then necessary to adopt a system of construction suitable for softer

or less stable ground, and especially so if the wall has to be built

upon a firm stratum overlying sand, as a diminution in the thickness of

the bed may cause a settlement and the wall to be pushed forward; for

there is always danger in sandy soils, unless the superimposed layer is

of considerable thickness, that the latter may be pierced and the whole

site be in jeopardy from a flow of the sand.

It may be that a dock must be built at a particular place, although the

ground may be full of springs, the beds inclined, or a level stratum

overlies an inclined bed of rock or softer earth incapable of standing

at any but a flat slope, the whole being an unstable mass, for the

deposits may extend to such considerable depths that to go below them is

impracticable; however, uniform weighting and consolidation by the

deposition of firm soils having insoluble particles will generally make

it firmer, and by compression and admixture will ultimately cause the

ground to be more solid. It should be loaded as long as possible, and

the material be deposited beyond the site so as to prevent the outside

and less consolidated earth slipping upon the works, and as it will be

disturbed by the loaded portion having subsided it may require slight

support upon the lower portion of the slope. When the settlement over

the base and the slopes to the original bed is equal and regular and

care is taken that the soft soil cannot run, by a due protection of its

surface it can be built upon. The rise of the mud in front and for some

distance from the wall is generally from about one-third to one-half of

its subsidence when loaded. The rate of sinking and uplifting of the

soil should be noticed, and whether it is equable and the depth to which

settlement extends is the same over a considerable area and practically

ceases at a certain depth.

Where a dry dock had to be made upon soft fine sand, Portland cement

concrete has been successfully used to cover the whole site. In such

soils, to avoid the danger of the bottom being blown up when a dock is

empty, and has not the counterbalancing weight of a ship, arrangements

should be made so that sufficient water is let in to counteract the

uplifting force. If it can be avoided it is advisable not to erect a

graving dock upon very porous earth, as the expense of an impervious

lining, whether of Portland cement concrete or clay puddle, and of

keeping it dry will be considerable; and when unequal settlement takes

place a run of the soil may occur and a slip of earth cause it to become

fissured and separated, especially in an estuary or bay with headlands,

as natural springs may make the sand a quicksand. Much depends upon the

head, if the water comes from the adjoining hills it may be so great as

to prevent any but a strongly constructed dock, acting as a watertight

box, being successful. The bearing power of the soil will also be

practically the same whether the foundations are a few feet or at a

considerable depth, as the head will be sufficient to make the springs

issue, and therefore disturb the earth.

The drainage of the site requires to be carefully deliberated, and no

rules can be laid down, but the flow of all surface water and springs

should be controlled. The chief aim should be rather to prevent water

reaching the site than to allow it to be present and then pump it away,

hence it is well to have the deepest sump and all water-raising

apparatus, if possible, outside the area of the works of art and in the

direction of the land or river flow. The first operation in draining the

site is to construct the necessary surface drains around it and also the

sinking of a sump, cylinder, or wells where required, which to be

effectual must be some few feet deeper than the lowest level of the

works of construction; but the depth should be no more than is

absolutely requisite, as the greater the lift of a pump the less the

efficiency; care being taken that the bottom of the sump is sufficiently

low that water gravitates to it, and that the pumping apparatus is at

such a height that it cannot be “drowned.” When the soil is of so loose

a character that pumping will cause it to “run,” the bottom can be

closed and holes made in the ring and water pumped in through them, and

in addition coarse gravel or broken stone can be inserted to act as a

filter. The act of drainage may cause soil that has been full of springs

to be entirely free from them, and cannot but always tend to

consolidation, if only because of settlement and the interstices or

fissures being less occupied by water or being reduced in size. In loose

soils, especially at the commencement of pumping operations, it is

advisable that it be not very rapid, as a quick change of condition may

cause the ground to be disturbed too much, whereas slow pumping may

clear the water as effectually and enable the soil to settle to its new

state. The location of the sump should be at or about the lowest level

of the ground well away from any wall or the dock area, and on the line

of the greatest flow of the land waters or natural watercourse, thus,

dealing with them before they reach the site of the works.

In loose and almost all earth the foundations should be covered up

quickly so as to preserve the natural condition of the soil. It is also

important to offer every possible support to the earth to prevent

initial movement, and slight assistance may afford the required

strength.

Unless there are cogent reasons to the contrary, the excavation for

dock-walls is better executed in trenches, the frames of which being

struck as the work progresses can be used again; the wall can thus be

supported by the earth on either side, and no lateral thrust is brought

upon the wall until it has set, and therefore the danger of bringing,

perhaps, the greatest strain upon it when in an imperfectly solid

condition is obviated, the back and top of the wall being protected from

the weather, and any movement or slip of the earth prevented; hence a

wall should act as quickly as practicable as a monolithic mass, and the

material forming it be thoroughly bonded and set in Portland cement or

the best hydraulic lime mortar, or be wholly of strong Portland cement

concrete.

The trench system of constructing dock-walls, covered ways, and all work

below ground is the safest to adopt, and in loose soils or clay that

expands and is quickly affected by atmospheric influences, and where

there are buildings or the ground is weighted, the erection of side

walls in trenches, and then the completion of the invert or arch is

often the only way in which the work can be executed. Even should the

formation be rocky, and the beds be inclined towards the site, the short

15 to 20 feet in length trench system is the best to adopt; and it may

be advisable or necessary to work day and night in order that the ground

may be left exposed for the least possible time and as machine mixed

Portland cement concrete walls can be erected much quicker than masonry

or brickwork, in such situations it is the best material to use. The

excavation for the walls should be confined within the smallest limits,

but no under-cutting should be allowed in the excavation trenches, or

the portion undercut may strain the upper part and cause it to slip and

subside.

In Chapter VIII. soft soil or weak ground is referred to, and as in

dockworks this so frequently occurs, the necessity of making every

reasonable provision against undue settlement is apparent. In such

foundations the weight upon them should be as equable as possible, for

any excess of load will cause subsidence at that place, and a large

bearing area is required which may not be easy to obtain, except by

increasing the area at the back, as the face must be nearly straight, or

only slightly curved, to enable vessels to get alongside, but the wall

should not be the heavier in front as it will aid an overturning

movement. Any dredging operations should be effected at such a distance

that the stability of a wall cannot be impaired, or a slip of earth may

occur, as the bared soil may commence to “blow” or run consequent upon

the dredging having relieved the ground of weight and, perhaps, removed

an impervious covering. A row of sheet piles inserted before dredging is

commenced, if well supported in front, may be sufficient to prevent

movement.

In dockwork, the margin allowed to resist lateral strain being

considerable, walls seldom fail by being fractured from excess of

lateral pressure; but either by vertical or lateral pressure causing

subsidence or the whole of a wall to slide forward upon the foundations;

and not from defective form, workmanship, or materials; but from a

compressive strain upon the face of a wall caused by a force tending to

overturn it; and, as a rule, when a wall inclines towards the face very

little more pressure will upset it. Generally the foundations are

sufficiently firm to support the insistent load, and the chief cause of

failure or undue movement is principally the result of slipping forward

and the want of adhesion of the soil and frictional resistance of the

wall upon it. How can this forward movement of the earth be prevented?

Before proceeding to name some means that can be employed it is

advisable to remember the cardinal principles of the construction of

dock-walls to resist lateral thrust, because a structure may be so

designed as to induce a movement or a slip of earth. They may be briefly

stated to be as follows:—

1. That the maximum weight is not required in the front of a dock-wall

but at the back, in order that the centre of gravity may be as far

distant from the face as the exigencies of sound construction will

allow.

2. That the centre of gravity should be as low as possible consistent

with the due strength of the upper portions of a wall so as to increase

the resistance to an overturning movement.

3. To have an equal load upon the ground, and to guard against any extra

strain upon the foundations near the face of a wall consequent upon the

lateral thrust, which has a tendency to cause a wall to overturn.

4. Particularly to provide against any forward movement either by having

deep foundations, protective works at the base of a wall, or by other

means.

NOTE.—It is obvious, in order to counteract a sliding movement, that

the foundations should be at right angles to the face batter at the

base, and not be horizontal. These inclining inward foundations,

however, can only conveniently extend for a portion of the width in

very thick walls, but throughout in those of less thickness. When no

support is possible in the front of a wall, such as an extra depth

of foundations, an invert, or a firm connection between the lower

portion of opposite dock-walls, &c., it is advisable to counterfort

the wall.

5. That the wall should be as symmetrical in form as possible,

particularly upon a soft foundation, in order to ensure equal loading

and settlement, and that it subsides without a forward movement or

tilting.

6. That it should be homogeneous, especially when the strata dip toward

it, sufficient provision being made to prevent any lodgment of water at

the back.

7. That it must have the usual margin of stability above the sufficient

weight and mass to resist the calculated lateral thrust of the earth and

that caused by it being surcharged by buildings, goods, cranes, or

machinery, which may cause unequal and sudden loading and strain; and to

withstand in sidelong ground the additional lateral thrust caused by the

inclination of the strata.

8. That the face be sufficiently perpendicular to allow ships to lay

close alongside, and that it be of the required hardness to resist the

rubbing of vessels, and especially of barges.

9. That, when a wall is constructed of different materials, a complete

union is effected of all the parts, and anything tending to make it of

varying strength, unless exceptionally required, should be avoided; such

as the junction of lime concrete with Portland cement concrete, or the

employment of the hardest and strongest bricks or stones and their

connection by means of a weak mortar.

10. That the resultant of the vertical and lateral pressure of the wall,

whether surcharged or not, falls well within the middle third of the

wall at its base. If not, the foundations will be unequally strained.

11. That any mooring posts or bollards, gate or sluice chamber machinery

be so placed and secured that they cause no serious additional strain

upon a dock-wall.

12. In soils that expand, such as the clays, dry backing should be

provided of absorbent and even material capable of compression

sufficiently to relieve a wall of severe pressure from the expansion of

the earth.

13. That selected material be used for the backing, and that it be

raised in layers inclining in a direction opposite to that of the wall,

care being taken that it cannot become water-charged.

Having briefly referred to a few of the chief principles of construction

of dock-walls, the connection between them and slips and subsidence in

earthwork has to be considered.

It is seldom a dock-wall fails from insufficient mass or incorrect

design. The chief element of danger being imperfect foundations causing

a forward movement of the wall, which may either result in a bulge or a

complete advance of the whole section for a considerable distance, and

usually when this forward motion commences it extends for a long length,

and has an apex, and is not only of a disastrous character, but also

difficult to quickly and permanently restore to a state of stability.

The wall may be most carefully designed, and be amply sufficient to

resist lateral thrust due to the pressure of the earth and any load upon

the quay, and nevertheless, although as a wall it is perfect and

unimpaired, the whole mass from want of sufficient depth of the

foundations or hold in the ground frequently slides forward: and there

are few extensive docks that have been constructed in any but the

firmest earth that have been free from such a mishap. In order to limit

the extent of a sliding movement, it might be said, reduce the lengths

of the wall and make the unsupported from the front length as short as

possible, and in doubtful earth it may be advisable to abandon all long

straight walls and adopt the system of short jetties, with ample width

at their heads for unloading and loading operations, for a vessel to lay

alongside, for store-house room, and a double line of rails and the

necessary free action of machinery; so as to avoid the expense of slips

pushing forward the wall: but to do this may almost ruin the commercial

prospects of a dock, for naturally owners and captains of ships will

send their vessels where the greatest facilities are offered and the

impediments to movement the least. Local considerations in each case

alone can determine the shortest required clear length. However, in

designing jetties to dock-walls it is advisable to so construct them

that they may act also as face counterforts to a wall, and be located at

any apparently weak place so as to give support where it is needed.

The chief danger is from variation of the earth in the foundations and

the existence of thin seams of an unstable character, or fissures in

rocky soil allowing percolation of water, most difficult, if not

bordering upon the impossible, to discover unless by trial pits and

borings over the whole area of the site of a dock, which would be a most

unwise proceeding, as has been previously named in this chapter. The

marvellous labours of geologists and others have enabled an accurate

opinion to be formed of the locality and the depth at which water and

certain minerals are to be found and also to indicate the earths in

which fissures and seams are frequent; but it must ever be impossible

for man to absolutely state that no unstable seams or veins exist over a

considerable area of ground, more especially in the superficial beds

upon which a dock-wall has to be constructed, although the probability

of their presence may be determined with some degree of precision. The

importance of a complete examination of the ground of the site and

neighbourhood is imperative, and particularly of any cuttings and

embankments in the locality.

In Chapter II. many conditions of earth are referred to in which slips

and movement are probable, but that now under consideration is usually

caused by a wall resting upon seams or weak veins of unstable soil;

therefore, there is danger when a comparatively thin seam of soft slimy

earth interposes between the hard bed upon which the foundations may

rest and one below the vein; for, but a little additional moisture may

sufficiently lubricate the surface to enable the firm bed to slide upon

it, the frictional resistance being thereby so reduced that movement

results. Similarly, in the case of a seam of gravel overlying clay, the

firm gravel bed may slide upon the clay: the foundations should then be

carried down to the clay. Piles at the base driven into the gravel will

be of little use; in fact, for permanent work, they are now almost

abandoned except for jetties, cills, aprons, dolphins, &c., for

uniformity of support is very difficult to obtain in pilework and cannot

be proved to exist; but piles may be useful to help to sustain a

structure until it has taken its permanent bearing and is in possession

of its full strength. For purposes of permanent support they are

somewhat unreliable, for failures have happened because of their

weakness, and walls have consequently been overturned; their chief use

is as auxiliary temporary aids to lateral stability, such as confining

concrete or lessening the percolation of water. However, the earth may

be in such a condition that it cannot retain them in position when they

are strained, then they are useless for lateral support. Their

resistance to horizontal strain varies much with the nature of the soil

and not necessarily according to its cohesive power; for instance,

experiments have shown sand to afford the greatest horizontal

resistance; clay, less; and in loose ashes the power is further reduced.

When a clay stratum is thin and overlies gravel or sand, provided the

necessary precautions are taken that the sand does not “boil,” it is

better to have the foundations upon the underlying gravel or sand, as

the layer of clay may bulge and slide upon the gravel and thrust out the

wall. Should a stratum of clay be superimposed upon another of similar

character, it will always be in a damp state conducive to lateral

instability, as there will be two sliding surfaces, and the upper may be

forced forward and carry the wall with it, and should a seam of sand be

interposed it may become “quick” and flow away. In such cases the

foundations should extend to a safe depth in the lower clay stratum or

be below the unstable seam, and when the depth of silt or unstable soil

overlying a firm bed, as rock, is considerable, the well system can be

adopted, provided the bottom is levelled. As seams, weak veins, and

fissures are so frequently met with the examples might be continued

almost _ad infinitum_. To ascertain whether they exist and their

location is one of the primary precautions to be observed, as dock-walls

usually fail from bulging or slipping forward, causing fracture or

overturning.

Additional weight or increase of the thickness of a wall may not suffice

to arrest movement of the base or forward motion of the earth. To place

the foundations at a greater depth may be impracticable, although it may

be the best method of restoration; it then becomes necessary to insert

an invert or strut between the walls in a narrow dock, or in a large

dock to attempt to remove the unstable seam in front of the wall and

prevent movement of the vein by a curtain-wall, additional weight, or

other means of consolidation; or by removing the solid backing, draining

the back, and the erection of a timber platform instead of the earth, so

that it reposes at its natural slope without creating a thrust upon the

wall; or by having packed rubble filling instead of ordinary solid

backing so as to remove or lighten the lateral pressure upon it, by

draining the ground at the back; or by counterforts from the foundation

to the ground level, at the front of the wall, especial care being taken

to prevent their parting from the main wall.

The arched wall system is sometimes adopted for docks, but as it

requires a longer time to construct, and is obviously more liable to be

damaged from settlement and by pressure of the earth and the failure of

joints and weak places, solid walls of Portland cement concrete with a

hard face are more to be desired; as in such situations a monolithic and

equally resisting mass is required, weight and mass being of importance.

The relative bulk of materials for a certain expenditure should be

considered, as it may happen that greatly increased weight and mass may

be obtained for no extra expense by the employment of a certain

substance. Combinations of brickwork and masonry and concrete are being

abandoned in favour of one homogeneous material throughout, and no yet

known aggregate fulfils this condition for such work as Portland cement

concrete, owing to the difficulty of making secure and perfect joints,

although hard brick facing may be necessary to protect the face from

wear by the rubbing of vessels. By a judicious adoption of material for

a dock-wall in order to give it weight and mass, and sufficiently deep

foundations, neither counterforts nor other special means of protection

against failure may be required, and slips and movements of the earth

may be prevented.

In the case of a stratum of soft soil of considerable depth overlying a

firm foundation, dock and quay-walls upon arches, not exceeding about 30

feet span, have been successfully erected upon wells sunk in or to the

solid ground, when by reason of the cost it was impracticable to carry a

solid wall to the firm ground; and no slipping, sliding forward, or

subsidence has occurred.

To prevent movement of the earth at the back of a wall pushing or

fracturing it and being the cause of a slip, it may be well to briefly

state a few points to which attention should be especially directed.

In adopting two systems of design of a wall, it is advisable that the

change of form be gradual and not made abruptly, or cracks may occur

from unequal settlement or load upon the foundations, and also when a

concrete bed is used under a wall a lime concrete layer cannot be firmly

joined to one composed of more solid and unyielding material, such as

Portland cement concrete. Similar walls erected upon different soils are

not equally strained, as one earth may scarcely change from the effects

of air and water, and another may vary daily according to the weather.

To counteract any overturning tendency, a wall should have a large

bearing area under the face portion, all projections and heavy copings

should be avoided, or anything that increases the load near the face at

the top. Weight and width of base and considerable depth of foundations

are most important, for well designed walls when placed upon stable

ground have been pushed forward simply from an insufficiently deep hold

in the ground; and the load upon the earth must be within its safe

bearing power. A wall should be designed so that its centre of gravity

is as far from the face as practicable, and when it is constructed of

stone or brick it is necessary that the joints be as strong as possible,

and be capable of resisting varying strains. Should lime mortar be used

it is probable it will be partly washed out, and Portland cement mortar

should always be preferred upon the face, or water may percolate into

the wall and even pass through it to the earth and cause slips and

subsidences.

Counterforts erected upon the outer face of retaining walls are much to

be preferred to any placed at the back, for support at the face is what

is required: in the latter case there is always a chance of their

becoming separated from the main wall, although when so located at the

back they are considered to lessen the lateral pressure of the earth by

dividing it, but any reduction caused by friction of the soil against

the sides may vary so greatly, according to the condition and subsidence

of the earth, that it is prudent to disregard it, and it is well to

remember that the pressure may become intensified or uneven upon the

main wall by deflection and concentration caused by the counterforts,

the result being the creation of weak and unduly strained places; it may

therefore be better to uniformly increase the thickness of the wall.

Counterforts of triangular shape on plan have been adopted in order to

lessen the lateral thrust by directing it to the sides of the triangles,

their bases being against the wall. It is doubtful whether the thrust of

the earth so acts under the usual varying conditions of work. The

necessity of mooring vessels alongside a dock or quay-wall prevents the

adoption of face counterforts except to a very limited extent; however,

although the form of a ship permits of only a slight batter for the

upper half, the lower portion may be inclined for a few feet from the

bottom. As the adoption of a batter on the face increases the area of

the base of a wall and its frictional surface upon the ground, and tends

to lessen overturning movement and undue strain upon the face, a dock,

quay, or retaining wall having a batter is to be preferred to one with a

vertical face; in fact, it acts as a triangular front counterfort, with

the great advantage of being an inherent part of a wall.

In foundations of a sandy or silty character, a dock or quay-wall having

wells in the cross section to within a few feet of its base filled with

rammed light dry material, the width of the bottom nearly approaching

that of the depth of water, has been adopted in preference to a solid

wall of less thickness, as affording a wider base, greater resistance to

overturning, and a reduction of the weight upon the foundations.

A slip of the earth backing is frequently caused from insufficient

drainage and a consequent accumulation of water behind a wall, producing

such pressure that the friction of the weight of the wall upon the

foundations is impaired and the support in front insufficient to resist

a forward movement, therefore the back drainage should receive due

attention. When a dock-wall has been thrust forward by the earth it may

be a serious movement, likely to increase and culminate in the

destruction of the wall, or be merely a slight lateral settlement. Such

slips are usually of considerable length, and have an apex where the

greatest forward motion has occurred. The chief remedies consist in

securing the foundations, supporting the face, lightening or removing

the pressure of the backing, draining the backwaters and preventing

their reaching the wall. Provided a wall is intact and not damaged, but

solidly and horizontally pushed forward, it need not be taken down, but

if vertical subsidence has taken place, or a settlement, at the face

only, it is generally of serious importance, and it may be necessary to

remove and replace it.

When a wall bulges slightly from the pressure of the earth at the back

and then ceases to move, the ground behind may have become in a state of

permanent equilibrium, and the lateral thrust, which before movement was

too great, may be so reduced that in the altered position it may remain

stable, provided the void is filled with light porous backing, and the

drainage of the earth receives due attention. Counterforts of piles

reaching to the level of the ground in front of the face have been

adopted to arrest and prevent a forward movement, but when the ground is

much disturbed and in a loose state, it is unadvisable to drive piles

because of vibration which may induce further motion. Instead of timber

pile counterforts, trenches might be cut at intervals along the toe and

be filled with quick-setting cement concrete, the filling to follow the

excavation as closely as possible, and no unsupported earth to be

allowed to remain.

As improper backing often induces a slip and subsidence of the earth and

the failure of a wall, it is of importance that it should be carefully

executed, for many well-designed walls have failed or bulged from being

badly and hurriedly backed with unsuitable and soft retentive material

in a deteriorated condition, the result being movement of the earth.

Care should be taken that the material of which the backing is composed

will always maintain the slope of repose upon which the calculations

have been based that determined the dimensions of the wall; therefore,

it should always have an angle of repose which is not much affected by

moisture. It is important to remember that according to the nature of

the backing so will be the pressure upon a wall, depending principally

upon the coefficient of friction of the earth upon similar earth, its

cohesion, its friction upon the surface of the back of a wall, the

inclination of the layers, the general character of the soil and the

effect of moisture upon it; and no vegetable, decayed, or “made” earth

should be used for such a purpose, nor material which gives a varying

thrust according to the state of dampness or dryness, and the lightest

dry, firm, and stable earth should be preferred. When rubble backing is

adopted and is carefully packed, the lateral thrust may be greatly

reduced, as in some degree, instead of backing a dry wall may be

considered to have been built at the back of the face wall. Clean ashes

are an excellent material for filling damp places in a wall, because

they not only absorb moisture, but are light, and stand permanently at a

slope of about 1 to 1 TO 1¼ to 1, but they can seldom be obtained in

sufficient quantity to be used in considerable masses; also broken

bricks and burnt ballast, although heavier, make good backing.

Earth that is much affected by air or water, or that expands, contracts,

or fissures, such as clay, is not good for filling, and dry masonry

retaining walls should not be hacked with earth but with rubble, or in

countries with a heavy rainfall the wall will most probably be forced

forward.

Care should be taken to have plenty of weep-holes in a wall so as to

obviate any accumulation of water, and it is well if the back has a

rough face in order to increase friction and prevent cavities in the

backing down which water may percolate. During construction a few

weep-holes in a dock or quay-wall should always be provided, until the

water is to be let in to a dock, when it is best to fill them with

strong Portland cement concrete as water would proceed along them from

the face and accumulate at the back of a wall; the permanent drainage

being affected by other means.

In pulling down walls that have been built with offsets at the back it

will usually be noticed that the filling does not rest upon them, but

that hollows occur as the earth subsides; for this reason, a straight

batter at the back is to be preferred. No support from the weight of the

earth, which is supposed to be upon them, should be relied upon,

although it may temporarily exist.

The filling should be commenced at the wall, and the layers be so

deposited that their slope approximates to that of being at right angles

to the surface of the slope of the ground rather than parallel to it.

When thought desirable and the natural ground is solid, it can be

benched, and the backing be damped and rammed.

The backing of a wall may consist not only of an inverted triangular

piece of earth, but extend for some distance to the rear, and may have a

top surface at a considerable height above it; the wall will then have

to sustain a severe thrust. The angle of repose of the earth should be

ascertained and the filling be thoroughly drained, or it may be

disintegrated by moisture, the whole mass gradually become unstable, and

finally push the wall forward with great force.

CHAPTER XII.

NOTES UPON SLIPS OF EARTH, SUBSIDENCES AND MOVEMENT IN FOUNDATIONS

CAUSED BY “BOILS” OR AN UPWARD RUSH OF WATER IN LOOSE

EARTHS.—CONSIDERATION OF SOME PRECAUTIONARY AND REMEDIAL OPERATIONS.

With regard to “boils” in sandy soils and the general drainage of the

site of dockworks, operations should be commenced as long as possible

before the works of construction, so as to lessen the probability of the

occurrence of slips, subsidences, and movement of earthwork. In order to

reduce infiltration, it is well to make a trench round the area of any

ground that has to be excavated, which may embrace the whole site. A

system of drains and conduits should be established within it, and at

the lowest level a sump at a convenient position, and to a depth a few

feet below that of the work; it can then be made the chief pumping

station of the dock; but all drawing away or flow of the earth must be

prevented: and to obviate erosion of the drains, they may require to be

rough lined with an impervious covering such as clay. The sump should

consist of an iron cylinder with proper provision against a run of soil.

In all loose earths the pumping station should be some distance from

buildings or roads so as to avoid any settlement.

When a choice of sites exists and the position of a dock is not

absolutely fixed, it may be possible to have the foundations of the

whole work in one kind of earth; if otherwise, one portion may be stable

and another unstable, always treacherous, and liable to slip and

subside. In any case settlement is not likely to be equal, and therefore

a foundation which is well able to sustain an evenly distributed load

may yield from unequal strain and excess of lateral thrust. Consequent

upon the situation of docks, the superficial beds upon which they have

to be erected frequently vary in stability and reliability; and the

location being altered in any direction may result most seriously; the

earth upon one side of a comparatively narrow river being stable in

character and on the opposite bank most treacherous. Difficulties often

arise in foundations, especially in sandy soils, from making borings and

trial pits too near the important parts of the work; they should be made

as reasonably far away as is convenient. In boring, a sand flow may

occur, when it may be necessary to fill the bore-hole and sustain the

soil by a covering, or by consolidating the sand by means of Portland

cement, and then, perhaps, reliable operations may be continued. Borings

should be considered as unreliable if merely superficial; in any case of

importance they should only be trusted for the place where they are

made, and not as indicating the nature or condition of the soil over a

considerable area. When pits cannot be sunk, it is desirable that in a

suspectedly treacherous site the bore-holes should be at every 200 or

300 feet.

Excavating pits, using test-bars, and driving piles are some of the

methods of determining the character of foundations, but care should be

taken to ascertain in boring that boulders, or thin strata of hard

gravel, are not mistaken for solid rock. In sand, mud, or soft clay they

can be made by means of an iron pipe and the water-jet system.

Experience has proved that boring with an auger is not so reliable as

boring with a tube, such as is used for artesian wells. In the case of

augers, when boulders are encountered, further boring is usually

arrested in that place and another bore-hole has to be commenced. Trial

pits, where practicable, should be preferred to boring, and they should

be sunk to a depth considerably below the lowest level of the intended

foundations, and then they may do for sump holes for pumping operations.

In testing ground by borings, several should be made, as one hole might

encounter a boulder or some hard soil, such as indurated clay, and the

latter may adhere to the auger and arrest its progress; the specimens

then brought up, being crushed and pressed together, will appear to be

firmer than the actual condition of the ground.

Having briefly referred to the preliminary drainage, and some methods of

ascertaining the nature of the ground, the former to lessen, and the

latter to aid discovery of the character of any probable upward lushes

of soil: “boils” in foundations are more specifically considered. It may

be said that they generally proceed from an impervious top stratum being

pierced, thereby tapping the water in a pervious bed which may be

imposed upon another impervious layer; for the upper bed being excavated

removes the weight upon the lower strata and induces a flow of the

previously confined water.

The source of disturbance may be either from surface-water in the top

soil or from deep underground springs, depending in great measure upon

the extent of the catchment area and head-water level of the district,

as its quantity and uplifting pressure will be principally governed by

them. When the soil is in a delicate state of equilibrium it only

requires a slight deteriorating alteration of the normal condition to

initiate a movement. The disturbance of the ground may be merely

superficial, nevertheless its effect may be sufficient to start a

“boil,” although the chief cause potently exists at some considerable

depth and distance.

“Boils” produced by simply surface-water require one system of

treatment; if from underground waters, another remedy is necessary.

Every effort should be made to know the reason, trace the origin, and to

ascertain the power of the disturbing forces. It may be possible to

determine them, although generally it can only be done by deductive

reasoning and logical inference. In any event immediate action is

invaluable when a “boil” appears. Weighting the ground around a “boil”

with an impervious mass of clay, and the insertion of a stand-pipe will

indicate the level to which the water will rise, varying according to

the seasons, and will give an approximate idea of the head supply and

pressure; care being taken that the pipe does not become obstructed and

that the water has a free flow. It may also be ascertained by the

insertion of pipe-rods to different depths, and by noticing the effects,

such as the rapidity, the quantity and the character of the discharge.

There may be an appreciable difference in the height to which the water

rises in various stand-pipes; if so, its flow is obstructed or the

source is not identical. An examination of the colour and nature of the

suspended matter in the water and a comparison of it with the strata may

show its source, but it is not always reliable unless the same colour is

maintained for the lowest depths.

A perusal of Chapter II. will indicate some situations in which “boils”

may be expected. The conditions under which they may appear are so

numerous that it may be stated they generally occur in any situation

when a layer of loose soil has a superimposed bed of more or less

impervious boil upon it which is perforated by the excavation for a dock

or other work; and especially should a water-bearing stratum, such as

chalk, underlie a sand stratum; also when a water-bearing stratum is

superimposed upon sand which lies upon an impervious bed such as clay,

as water will percolate to the sand; and when sand overlies a

water-charged stratum; or at the outcrop of chalk hills near the site;

or where sand is below an impervious layer of clay and the latter is

tapped. The “boils,” of course, become more serious as the head of water

increases, and also when the strata dip towards the site.

An impervious bed may be of such thickness, that when the lowest

foundation is excavated there may be no fear of water from any

underground source being forced up through it; however, unless the

thickness of the upper layer is known to be nearly uniform over the site

of a dock, which is seldom the case, the water pressure, aided by the

weight of the structure, may separate or loosen the soil at a weak

place, and then a “boil” will be the result. The thickness of the crust

required, which may be anything from about 15 feet upward, may be

approximately ascertained by weighting the earth considerably above the

load it will permanently have to sustain, and by watching the subsidence

and general effect. Great care should be taken not to perforate the firm

stratum, as danger will at once ensue should loose soil, such as sand,

be tapped, for cavities will then be produced causing subsidence,

probable fracture of the firm stratum, “boils,” and slips of serious

extent. When a heavy structure has to be built upon such a soil, and it

is impossible because of the expense to place the foundations except

upon this superficial bed, experiments in the direction indicated should

always be made, and any other means of proof that circumstances may

allow other than by perforation of the stratum: in any case every effort

should be made to reliably ascertain that the firm stratum is of uniform

depth and character.

The discharge channel of any underground waters likely to disturb the

foundations should be discovered, and when there are adjacent hills, by

tracing their dampest part the probable direction of the surface flow

may become known. Inquiry should be made to find out whether any borings

have been made over the site in order to determine if the “boils” have

been artificially created or are natural blow-wells, in which the water

rises over the top and can, perhaps, be led away by gravitation. It is

advisable to fill every bore-hole with Portland cement mortar before any

excavation is commenced, or they will burst out when the surrounding

ground is disturbed. Bags containing shot have been used to help to

close a hole, but a preferable method is to insert a pipe down the bore,

it having been previously cleared of all dirt; and to fill it with neat

Portland cement mortar, the tube being raised as the hole is filled.

Such a method, although successful with small holes, is useless in the

case of “blows” of the ordinary size. A permanent shaft or cylinder is

then necessary.

With regard to the treatment of “boils” in foundations, when in the

possession of the information hereinbefore named it may indicate a

remedy either for “boils” in which the cause of disturbance is at a

considerable depth and the head and fall of water moderately large, or

when the agitation is merely superficial, which may result from the

range of the tidal waters or the want of surface drainage.

When a “boil” is of the first order, it is useless attempting to simply

stop it by force and prevent the issue of water, as such a method of

procedure would result in merely diverting the disturbance, and in

addition leave a weak place in the foundations; however, if many “boils”

appear, and the disintegrating agency of the water is removed either by

conducting or diverting the flow from the site, they can then be filled,

and it should be simultaneously effected, or any mere exudation in the

whole of the “boils” may become increased in one or two to a flow

sufficient to cause a slip or instability, and any concentration of the

discharge may produce movement of the ground. Small springs or “boils”

have been sufficiently arrested by depositing clay over them, excavating

it, and putting in the foundations very promptly and before the “boil”

burst out again; such treatment, however, may lead to the backing of a

wall becoming saturated and the water being dammed up: on the other

hand, the “boil” may be sealed and the water flow away in the original

underground channels; also when a “boil” is tapped it may induce an

increased flow of the underground waters, for they will find the course

of least resistance; and should they reach a fine sand stratum will

filter through and set it in motion, provided the head is sufficient to

overcome friction; and the support rendered to any overlying stratum

will therefore be destroyed.

In the event of there being only one “boil,” but that of important

extent, perhaps the best way to proceed is to place clay upon the

surface for a few feet around it, the weight of which should not be less

than the normal pressure of the earth removed by excavation, _vide_

Chapter V.: and by the insertion of a cylinder, with a properly designed

bottom, to prevent the issue of soil to a level some feet below the

lowest intended excavation upon the site, or to the depth at which the

greatest flow of water is obtained; care being taken that no run of sand

or soil is allowed and that the water is not charged with earth, either

by a thick gravel layer until the sand ceases to pass through it, or by

other means, remembering that the motive power of running sand is that

of the pressure of water. The flow should be gently led away, if

necessary, to a sump, and discharged by pumping, thus draining the

works. The sump should always be lined, not only to prevent any flow

upon or into the surface causing saturation, but so as to control and

regulate the water some feet below the lowest level of the foundations,

the great point being to draw downwards the water from the earth and

then to carry it away. An iron or wooden cylinder sump is to be

preferred to one constructed of sheet piles or timber-lined, as the soil

may be so loose that any shaking or vibration should not be allowed, and

a close joint is not easy to attain in timber pile work. It is well to

deposit some clay puddle for a little distance round the sump after

sinking it in order to weight the soil and steady the cylinder. When a

discharging outlet has been made it should be maintained in perfect

condition, or a slip or subsidence may occur from leakage or diversion

of the flow. It may also happen that no discharge is necessary, for if

the “boil” is only, as it were, of local extent and caused by a weak

vein in otherwise stable soil, the water may rise to the head level and

there remain; the equilibrium being restored which had been destroyed by

reason of the upper strata of an impervious character being excavated.

The removal of the normal pressure upon the loose soil to some extent,

liberates the water, and the head ceasing to be balanced by it, the

earth cannot do otherwise than allow water to upraise it. A calculation

of the weight of the earth removed may give an indication within which

will be the probable head level of the supply, and usually it is much

below a head of water that would equal the normal pressure of the soil.

Supposing the weight of the earth to be twice that of water, when 10

feet of soil is excavated before a “boil” began, its removal would

approximately be equivalent to a 20 feet head of water.

The great point is to confine a “boil” within a certain space, and there

to treat it and so prevent the appearance of others and interruption to

the prosecution of the works. All agitation of the surface of sand,

especially when a “boil” appears, must be prevented, as inducing a

disturbed and more porous condition.

When the seat of a “boil” must be built over, the well system of

foundations can be used as not only affording reliable support, but also

preventing a “boil” extending, and allowing the flow of water to be

discharged, care being taken that complete connection is made with a

wall constructed upon any other principle. If “boils” appear over a

considerable area, it is evident that the disturbance is general, and

may either be because the site or the foundations of the works are below

the level at which water is usually found in the district, or be

produced by the rise and fall of tidal waters. When by the former,

drains are required a few feet below the general water-bearing level,

and they should intercept the percolation of the land waters and conduct

them from the site of the works, or so gently localize them that they

can be treated as before described. The land waters are the most

troublesome because their volume, time of appearance, and duration of

flow will vary, whereas the range of the tidal waters is known, and

therefore the effects may be ascertained with some approach to accuracy.

It may be impossible to erect walls in very loose soils without

encircling the site with sheet piles to prevent a run of sand, to lessen

“blows” and to reduce the disturbance, as placing them upon an

impervious stratum maybe prohibitory because of the expense: but the

adoption of the well system of foundations down to a firm stratum, or at

a sufficient depth to be stable, and the building walls thereon may

cause the temporary pile support to be unnecessary and prevent a slip or

subsidence. Quick erection and the least exposure or disturbance of

earth are essentials in such cases.

By the use of Portland cement concrete, foundations can now be cheaply

obtained with the ground in a wet condition, which in the days when it

was considered only suitable for secondary works, would have required

the employment of expensive methods of construction. It may be unwise to

arrest the upward flow of the tidal waters in loose soil except by

intercepting, lessening, or preventing it by shore protection, for the

upward pressure of the head of water may “blow” up the earth and cause

disturbance; on the other hand, should the “boils” be merely superficial

and the head very little, a thick watertight covering of Portland cement

concrete, when accompanied by the necessary drainage operations, may

suffice; the surface water blows being covered, and the pressure of the

water being overbalanced by the superimposed weight.

All works to prevent “boils” or to drain waters from the site should, if

possible, be executed prior to the excavation, as the effect of draining

can then be observed and whether any “boils” arise from land waters or

from a tidal flow, and the earth will have had some time in which to

subside and become consolidated. A month’s test will afford some

criterion in a variable climate and when changes of weather are

frequent; but should a long period of drought prevail it may be no

indication of the state in which the earth would be in a wet season, or

under ordinary meteorological influences; and further, the land waters

may take a considerable time in flowing to the shore, depending upon the

catchment area, character of the soil, position of the beds, whether the

district is drained or not, and many other local conditions.

When the soil is firm clay, the locality of any leakage or water seam in

it will usually be indicated by surface exudation over the water-charged

vein; also in nearly all compact earth, particularly if of a clayey or

loamy nature, and therefore the seat of disturbance may not have to be

discovered as in looser or sandy soils.

Should a water-bearing fissure occur in rock and it be necessary to stop

it, there being no danger of diverting the flow to another place upon

the site of the works, it can be done by means of neat Portland cement

mortar pressed down and kept in place by timber strips, weights or other

means until it has set; the face of the fissure should be made as clean

as possible, and the water-bearing seam be filled and weighted directly

a leak appears, or the flow may become much increased and accelerated

and a current of water be induced which may become of serious extent.

To avoid slips and movement of the ground when it is necessary that a

water-charged bed of sand be excavated to enable a structure to be built

upon a firm impervious stratum below it, sheet piles can be used or

corrugated iron sheets, strutted at intervals, so as to prevent a run

and slip of sand; and the space between them can be excavated, a

Portland cement concrete foundation being inserted upon the firm soil,

the wall being erected in lengths, and the piles or case being

practically three-sided so as to leave no unsupported surface of sand,

care being taken to prevent any run between the front and the back of

the last built length of wall and the earth, so that each length is

erected in a contained area.

INDEX.

A.

Absorption, broken earth, 70

——, chalk, 29

——, sand, 34

Alignment of public works, _vide_ location, 9–12, 146

Alluvial deposits, safe load upon, 88–89

——, soil, slope, 100

——, protection of the banks of rivers, 126

Angular particles in earth, 36

Approaches to culverts, 84, 85

Arched system, dock-walls, 210, 211

Argillaceous rocks, slopes, 24

Ashes, slope, 100

Atmospheric pressure, influence of, 17

B.

Backing, dock-walls, 214, 215

Ballast, upon soft soil, 82, 151

Basalt, 18

Beach, artificial slope, 180

——, natural slope, 100, 101

Benching ground, 160, 162

Blue clay, 38, 89

Bog-land, safe load upon, 88

——, sustaining power, 152

——, treatment of, 149–151

“Boils” in foundations, ascertaining character of, 218, 219, 224

——, disturbance of soil, 218, 220, 221

——, origin and source, 218, 219, 223, 224

——, treatment of, 216, 218–225

“Boils” in foundations, well system of foundations for, 223

Borings, effect of, 217, 220

Boulders, 22

Breakwaters and piers, foundations of, 181, 182

Brown clay, 13, 42, 43

Bulging of ground, 147, 148

——, of walls, 207, 213

Burning, slipped earth, 62, 63

C.

Canal and similar earthworks.

——, admittance of navigation water, 190

——, clay and loamy soils, 190

——, concrete wall, 193

——, consolidation, 190–192

——, drawing off water, 188, 196

——, failure of, 8, 194–196

——, leakage in, 189

——, peat and bog, 187

——, precautions, 190, 193

——, preparation of foundations, 191

——, protection of surfaces, 188, 192–194

——, puddle wall, when objectionable, 192

——, sand, 187

——, selection of earth, 191, 192

——, —— location, 146, 190

——, time of occurrence of slips, 190

Catchwater drains, 76, 77

Causes of slips and subsidences in earth.

——, principal, 8

Causes of slips and subsidences in earth, universality, 1

——, variety of, 1

——, water, the chief cause, 8

Causes of slips in cuttings and embankments, enumeration thereof, 2–8

Chalk, absorption of, 29

——, affinity for water, 28

——, angle of friction of water in, 28, 29

——, characteristics of, 30

——, classification for earthwork purposes, 31, 32

——, cohesion of, 31

——, cuttings in, 33

——, difficulty of draining, 29

——, discharge of water in, 29–30

——, faults, crevices and pot-holes, 28

——, flints in, 28

——, magnitude of many slips in, 32

——, mutability of, 29

——, non-water bearing, 30–31

——, pressure on, effects of, 33

——, safe load upon, argillaceous, 89

——, —— white, 89

——, separation, danger of, 28

——, slopes, 27, 100

——, treacherous condition, 31

——, —— soil, 13

——, treatment of springs in, 29

——, tunnel entrances in, 32, 33

——, use as drains in _other_ soils, 28

——, varieties of, 26, 27

——, veins and seams in, 29

——, water-bearing level in, 30

Classification of earths, 23, 36, 46

Clay, beds of, 41, 42

——, characteristics of, 37, 38, 40–43

——, classification of, 46

——, condition of stability, 39

——, contraction of, 38, 39

——, expansion of, 37, 38

——, fissures and cracks in, 38, 39

——, humid, 41

——, load upon, blue, 89

——, —— damp, 89

——, —— diluvial, 89

——, —— intermixed beds, 89

Clay, load upon, muddy, 88

——, —— solid, 89

——, —— upheaved beds, 89

——, —— wet, 88

——, —— yellow, 89

——, mud mistaken for solid clay, 42, 43

——, notes upon, blue, 38, 39

——, —— boulder 13, 42, 43

——, —— brown, 13, 42, 43

——, —— gault, 43

——, —— lias, 13, 39

——, —— marl, 45, 46

——, —— mica in, 43

——, —— red, 43

——, —— rock, 22

——, —— sandy, 39–41

——, —— slate, 18

——, over-draining it, 39

——, protecting the surface of, 40

——, slopes, 14, 46–49, 100, 108

——, testing character of, 40, 41

——, time of slip in, 43

——, underground excavation in, 49

——, weakness in, 43, 44

Cohesion of earths, general consideration, 14, 105, 106

——, values, 106

Concrete blocks, slopes of, 101

Condition of earth, worst, 16

Configuration of ground, influence of, 18

Consolidation, soft soil, 148, 149, 153, 154

Continuous surface, value of, 87

Counterforts, earth, 57, 58, 61, 121

——, in slopes, 119–122

Covering, nature of, 122, 123

——, slope, 122

Crumbling of the surface, 2

Crystalline rocks, 21

Culverts, approaches to, 84, 85

——, design of, 85

——, failures of, 195

——, form, 83, 84

——, general considerations, 83, 84

——, influence on slips and subsidences in earthwork, 83

——, special systems, 85, 86

Cuttings, causes of slips in, 2–5

——, side, 157

——, slips in, require separate consideration, 2

——, vibration in, 136–141

D.

Deposition of embankments.

——, effect of, 161

——, failure of, 167

——, form of tip head, 168

——, general, 156, 157, 159

——, height of tip, 163, 164, 165, 166, 167

——, junction of high embankments, 164, 165

——, long lead, 163

——, method of, on soft soils, &c., 154, 155

——, precautions in the, 157, 158, 161, 162

——, preparation of ground, 159, 160

——, from side-cutting, 157, 158

——, side tipping, 167, 168

——, specifications, impracticable, 156, 157

——, steam navvy and the, 169, 170

——, systems of, 159, 161–164, 166–169

——, weather influence, 167

Depth, influence of, in cuttings, 87

——, —— embankments, 87

——, —— foundations, 89, 90

Diluvial soil, safe load upon, 89

Diversion of land waters, 15

——, —— rivers, streams &c., 196, 197. _Vide_ RIVER.

Dock earthworks.

——, ascertaining nature of soil, 198, 199, 208–210, 217, 218

——, “Boils” in foundations, 216. _Vide_ “BOILS.”

——, causes of slips and subsidences in, 3–8

——, difficulties of construction, 198–200

——, ditches and watercourses, 201

——, drainage of site and pumping of water, 202, 203, 216, 224

——, execution of, 203, 204

——, foundation, 201

——, liability to slip, 199–201,204, 205

——, location of, 200–202, 216, 217

——, pressure of water in, 199, 200

Dock-walls.

——, arched system, 210, 211

——, back drainage, 213, 215

Dock-walls, backing, 214, 215

——, bulging, 207, 213

——, construction of, 205–207, 211–213

——, counterforts to, 212

——, jetties as face counterforts, 207, 208

——, material, 203, 210

——, piles, use of in, 209

——, Portland cement concrete wall, 223

——, prevention of movement in, 207–212

——, repairing a slipped wall, 213, 214

——, sliding forward of, 207, 208, 213

——, well system of, 223

Drainage, ballasting, 82

——, catchwater drains and ditches, 76, 77

——, chalk, 29

——, clay, 73–75

——, controlling discharge, 74

——, counterforts and trench drains, 77–80

——, culverts, 83–85

——, equable, necessary, 150

——, field drains, interception of, 77

——, filling of trench drains, 78, 79

——, formation, 81, 82

——, mixed soils, 72

——, number and depth of drains, 75

——, object of, 65, 72, 73, 81

——, overflowing earthworks, 74, 75

——, peat, 77, 149, 150

——, sand, 77, 80, 81

——, seat of an embankment, 82, 83, 85, 86

——, slope drains, 77–80, 111, 112

——, soft soils, 73, 149, 150

——, spoil banks, 80

——, springs, 75, 76, 80, 81

——, systems required, 72–77, 80, 81

——, well system, 81

Drifts of snow, protection against, 131–134

Drift soil, treacherous, 13, 18

——, —— upon sidelong ground, 143–145

E.

Earths, preservation when loaded, 88

——, maximum safe load upon, 87–91

——, table of weights of, 92

——, treacherous, 13

——, universality of slips in, 2

Effect of a slip, cuttings, 50

——, embankments, 50

——, influence of depth of cutting and height of embankment, 50, 51

Embankments, ballast upon, 151

——, causes of slips and subsidences, 2, 5–8

——, condition of earth, 97

——, consolidation of soft soil, 148–151, 153–155

——, consolidation by time, 17

——, deposition of, 156–159, 161–9

——, deposition, from side cutting, 157, 158

——, depositing upon soft soil, 154, 155, 168, 169

——, deterioration of excavated earth, 93, 94

——, drainage of the seat of, 160, 161

——, failure of, 167

——, fascine mattress bed, 155

——, forming, 158

——, height, safe, 91, 92

——, —— of tip, 164–167

——, high, 95

——, homogeneousness, 158

——, junction of, precautions, 164, 165

——, long lead, 163

——, precautions in deposition, 157–159, 161–163

——, preparation of ground, 159–161

——, safe maximum heights of, 94–96

——, safe load upon, 88, 93, 94

——, sidelong ground, 142, 143, 145

——, side tipping, 167, 168

Embankments.

——, soft soil, 147–149

——, specifications, impracticable, 156, 157

——, steam navvy, influence of, 169, 170

——, systems of deposition, 159, 161–169

Embankments, tip head, form of, 168

——, vibration, effects of, 136–141

——, weather influence during deposition, 167

Estuary or shore embankments.

——, closure of, 177

——, deposition of, 176–177

——, design, 171–173

——, effect of erection, 173, 179, 180

——, excavation for, 176, 177, 187

——, height, 179

——, material, 187

——, pressures, calculations showing changes of 182–187

——, protection of slope, 174–180

——, shore deposit protection, 176, 177

——, site, 171

——, slope, form of, 174, 180

——, trestle work, and on sea-shore, 180

——, viaducts and, 172, 173, 180

F.

Fascine covering of slopes, 128, 129, 178, 179

Field drains, interception of, 77

Fissures in slopes, 127, 128

Flint, beds in chalk, 28

Flood-waters, protection against, 57

Foot of slope, general considerations, 115

——, protection and preservation, 115–131

Formation, special form of, 159

Formation width, ballast, 151

——, influence of, 134, 135

——, junction of embankments, 136

——, lateral settlement, 135

——, severe climates, 134, 135

——, sidelong ground, 143

——, temperate climates, 135

——, value of width, 134, 135

——, wet cuttings, 134

“Forming” embankments, 158

Friable rock, slopes, 24

——, chalk, slopes, 27

Friction of earths, effects of water on, 103

——, general considerations, 101–103, 105, 106

Friction of earths, necessity of relying upon, 105, 106

——, relation to slope, 102

——, table of slopes calculated from coefficient of friction, 104

G.

Glacial deposits, treacherous, 18

Gneiss, slopes, 23

Granite, slopes, 23

Grass, covering slope with, 122–124

Gravel, angularity and roundness of particles, 36

——, ascertaining character of, 36

——, boulder, and gravel and sand, safe load upon, 89

——, capability of subsidence, 35

——, classification for earthwork purposes, 36

——, compact, safe load upon, 89

——, counterforts, 119

——, interstices in, 35

——, slopes, 37, 100, 101, 106, 107

Ground, configuration of, 18

Groynes, protecting slopes by, 174–176

H.

Height, of embankments, 91–97

——, influence of length on safe, 95

——, influence of nature and condition of earth, 97

——, limit of, in embankments, 91–96

——, table of approximate safe maximum heights for different earths,

94–96

——, usual limitations, 94, 95

I.

Igneous rocks, slopes, 23

Impairing elements, variableness of, 1

Inclined beds, danger in rock, 20, 21, 26

Instability, earth’s crust, 1

Irrigation earthworks, special causes of slips and subsidences, 187.

_Vide_ ESTUARY EMBANKMENTS.

J.

Jetties, as counterforts to dock walls, 207, 208

——, foundations of, 181, 182

Junction, of high embankments, 136, 164, 165

L.

Land drains, cause of slips, 15

Land waters, diversion of, 15

Lateral settlement, in embankments, 135, 136

Lead, effect of long, 163

Lengths, short excavated lengths, when necessary, 33, 143, 203, 204,

225

Limestone, 18, 24, 25

Load on earth, continuous surface, 87

——, detached portions, 87

——, experiments to ascertain safe, 87, 88

——, influence of area, 87

——, influence of normal load, 90, 91

——, safe, 87–90

Loamy soils, slopes, 37

——, safe load, 89

Location of earthworks, configuration of ground, 10, 12, 146

——, drift soil, 9, 11, 146

——, obstructing drainage, etc., by embankment, 9–11

——, river near to, 10

——, river and stream diversions, 10

——, road, near to, 10

——, road approaches, 11

——, stations, position of, 10

——, trestles or solid embankment, 12

——, tunnels or cuttings, 11

——, valley or hill-side line, 11

——, valley, stable side of, 10

——, valley, sunny or wooded side, 11

——, wetter side of a hill, 12

Loose chalk, 27

Loose rock, 24

M.

Maintenance, soft ground, 151

Marly chalk, slopes, 27

Marly soils, characteristics of, 44–46

——, difficulty of draining, 46

——, safe load upon, 89

——, slopes, 100

Marsh earth, slope, 100

Marsh land, safe load upon, 88

Metalling, slope, 101

Mica-schist, hardness of, etc., 24

——, quantity of quartz in it, 24

Mixtures of earth, troublesome, 20

Moraine, treacherous soil, 18

Morass, safe load upon, 88

Mud, consolidated, slope, 100

Mutability, of the created elements, 1

N.

Normal load, influence of, 89–91

Normal pressure, consideration of, 91

Number of drains, 75

O.

Object of, drainage, 72, 73, 74, 81

Obstruction, of drainage by embankment, 9, 10, 11

Open trestle work _versus_ embankments, 180

Overflow, drainage by, 75

P.

Peat moss, safe load upon, 88

Peat soil, consolidation of, 153, 154

——, drainage of, 149–151

——, nature of, 152, 153

——, necessary test, 151, 152

——, sustaining power of, 151, 152

Peat turf, slope, 100

Percolation of water, overflow and requisite pumping, 69, 70

——, chalk, 70, 71

——, clay soils, 70

——, cuttings and embankments, 65–67

——, effect of, 66–69

——, excavated earth, 70

Percolation of water, fissures, 69–71

——, loamy soils, 70–72

——, marly clays, 70

Percolation of water, maximum, 66, 68

——, principles of, 67–72

——, regulation of, 65, 66

——, sand, 71

——, top soil, 70

Piers, foundations of, 181, 182

Pitching, on unequally consolidated soil, 131

——, precautions respecting, 130–131

——, protection of slope by, 129, 130

——, when necessary, 129, 130

Porphyry, slope, 23

Pressure, normal, on earth, 91

Probability of a slip, 13–18

Protective works, burning the slipped earth, 62, 63

——, cover shed and catch trenches, 59, 60

——, different methods necessary, 52–60

——, earth counterforts, 57, 58, 61, 121

——, enumeration of, 53–55

——, exceptional, 55, 57, 58

——, flexible _v._ rigid drains, 56

——, flood waters, 57

——, general principles, 55–58

——, object of, 52

——, salifiable earth, 59

——, slopes, general considerations, 115–117

——, trickling of surface soil, 58

——, turf and plant, 122–124

——, weighting the foot of, 119

——, when drainage alone required, 56

——, when required, 52

——, where to commence drainage operations, 56

Q.

Quartz, slope, 23

Quicksand, safe load upon, 88

R.

Rainfall, influence of, 15, 16

Rammed earth counterforts, 61, 121

——, thickness of layers, 120

Reclamation embankments. _Vide_ ESTUARY EMBANKMENTS.

——, special causes of slips in, 8

Removal of slipped earth, general considerations and treatment, 60–64

Repairing slope, 121

Reservoir embankments, 187, etc. _Vide_ CANALS.

Retaining walls, desirability, 117, 118, 146

——, draining back of, 117

——, failure of, when probable, 118

——, form, 118, 119

——, protection to slope, 115–119

River banks. _Vide_ CANALS, 187, and

——, ——, ESTUARY EMBANKMENTS.

——, protected _v._ unprotected slopes, 48, 49

——, slopes of, 48

——, tree protection of, 126, 127

River diversions, 196, 197

Road metalling, slope, 101

Rock, ascertaining character of, 18–21

——, character, etc., 18–22

——, classification for earthwork purposes, 23

——, crystalline, 21

——, examination of, 19

——, general consideration, 18–26

——, inclined beds, 20, 21, 26

——, mica schist, 24

——, nature of some, 21, 22

——, safe load upon, 90, 91

——, sandstone, 90

——, schistose, 24

——, slopes, friable, 24

——, ——, loose, 24

——, ——, sedimentary, 23

——, stratified, 18–20

——, treacherous condition of, 8, 9

——, unevenness of surface, 18, 19

——, unstratified, 18

——, variation of character, 18, 19

——, water levels in, 24

Rubble mound, safe load upon, 91

——, slope, 101

S.

Salifiable earth, 59

Sandstone, 19, 22, 25

Sandstone, slope, 23, 24

——, in tropical climates, 25

Sand, absorption of water, 34

Sand, angularity _v._ roundness of particles, 36

——, capability of subsidence, 35

——, compact, safe load upon, 89

——, delicate state of some, 34

——, drainage of, 34

——, drawing away of, 33

——, excavating in short lengths, 33, 143, 203, 204

——, firm, safe load upon, 89

——, interstices in, 35

——, light sand, protection of, 123–125

——, loose sand, safe load upon, 89

——, mutability of condition, 34

——, sand “boils” in foundations, _Vide_ “BOILS.”

——, silty, safe load upon, 89

——, slopes of repose, 36, 37, 100, 106, 107

——, soft sand rock, 90

——, springs in, 33

——, superficial beds of, 89

——, when likely to slip, 34

Sandy clay, 39, 40

Schistose rock, 24

Seat of embankment, drainage, 160

Setting out, with reference to earthwork, 9–12

Settlement, lateral, of an embankment, 135

——, provision against in slopes, 125, 126

Shales, characteristics of, 20, 21, 44

——, iron pyrites in, 44

——, safe load upon, 89

——, slopes, 21, 100

Shore embankments. _Vide_ ESTUARY EMBANKMENTS.

Sidelong ground, cuttings and embankments upon, 142–146

——, depositing embankments upon, 145, 146

——, excavating in short lengths, 33, 143, 203, 204, 225

——, object of precautionary works, 142–145

——, precautions, 142–146

——, when inclined rock, 146

——, when especially treacherous, 144–146

——, varying the slope, 146

Silt, safe load upon, 88

——, slope, 100

——, treacherous, 18

Slake, safe load upon, 88

Slate rock, 21

Slipped earth, treatment and removal of, 60–64

Slips in canals, special causes, 8, 187

—— time of occurrence, 190, etc.

—— docks of occurrence, 187

—— drainage works, 187

—— irrigation works, 187

—— reclamation works, 187

—— waterworks, 187

Slips in cuttings, causes of, 2, 3, 4, 5, 8

——, embankments, causes of, 5–8

Slips, immediate action necessary, 61

Slopes, bare, when can be left, 125

——, cess upon, 111, 113, 114

——, counterfort at foot of, 119–122

——, curved, 109–111

——, disturbance of, 111, 112

——, fascine covering, 128, 129

——, fissures in, treatment of, 127–128

——, flat, should not be too, 98

——, form of, 107–108, 110, 111

——, general considerations, 98–99, 105–110

——, nature of covering, 122–131

——, object of protection, 119, 122

——, permanent, of earth, 98–101, 106–109, 146

——, precautions concerning tree protection, 126, 127

——, protection and preservation, general considerations and different

systems, 115–131

——, protection against burrowing animals and crustacea, 127

——, protection of light sand, 123–125

——, protective coverings, 122–129

——, provision against settlement, 125, 126

——, range of slopes in different earths, 100, 101

——, repairing, 61–64, 121, 122

——, sand veins in, 120

——, steepest, 112, 113

Slopes, stone pitching upon, 129–131

——, terraced, 111

——, time to trim, 125

——, trees and shrubs upon, protection by, 126, 127

——, trench drains in, 77, 78, 111, 112, 121

——, trimmed as excavated, 111

——, turfing, grass and plant protection, 122–127

——, varying the form of, 109, 110, 111

——, walls, 117–119

Snow-drifts, prevention of, protection against, 131–134

Soft soil, consolidation of, 153, 154

——, depositing embankments upon, 154, 155

——, drainage, 73, 149–151

——, embankments upon, 147–149

——, equable drainage necessary, 150

——, influence of ballast on formation, 151

——, sustaining power of, 151, 152

——, testing, 148, 149, 152

Springs in chalk, treatment of, 29

Subsidence, capability of sand, 35

—— in soft ground, 151

Subsidences in embankments, causes of, 5–8

T.

Time of occurrence, slips, 13–17

——, usual, 15–17

——, submerged and unsubmerged work, 17, 189

Trap rock, slopes, 23

Treacherous earths, 13, 204, 217

Tree protection, general, 16, 126

——, precautions concerning, 126, 127

——, slopes, 126, 127

Trickling of surface soil, 58

Trimming slopes, 111, 125

Tunnels, entrances of, in chalk, 32

Turfing slopes, 122–126

U.

Universality of slips, 1, 2

Unstable soils, 13, 204, 217

Upheaval of ground, 147, 148

V.

Variableness of, impairing elements, 1

Variety of causes of slips and subsidences, 1

Viaduct, _versus_ embankment, 171–173, 179, 180

Vibration, in cuttings and embankments, 136, 137

——, deteriorating influence of, 136–141

——, effect on earths, 136, 137

——, influence of, 8, 9

——, lateral pressure and, 137

——, test of effects, 141

W.

Water, the chief cause of deterioration of earth, 8

Water-bearing level, 14, 24

Waterwork earthworks, special causes of slips and subsidences in, 8,

187. _Vide_ CANALS, etc.

Water pressure in cuttings, 14

Weights of different earths, 92

Widening works, consideration respecting slopes, 112–114

——, steepening the slopes, 112–114

Y.

Yellow-clay, 43, 89

LONDON: PRINTED BY WILLIAM CLOWES AND SONS, LIMITED, STAMFORD STREET AND

CHARING CROSS.

------------------------------------------------------------------------

Crown 8vo., cloth, price 4s. 6d.

NOTES ON CONCRETE AND WORKS IN CONCRETE.

BY JOHN NEWMAN, _Assoc. M. Inst. C.E._

REVIEWS OF THE PRESS.

=ENGINEERING=—_11th November, 1887_.

“_An epitome of the best practice which may be relied upon not to

mislead._”

“The successful construction of works in concrete is a difficult matter

to explain in books.”

“All the points which open the way to bad work are carefully pointed out

by our author with a pertinacious insistence which demonstrates his

clear appreciation of their value.”

=IRON=—_21st October, 1887_.

“As numerous examples are cited of the use of concrete in public works,

and details supplied, _the book will greatly assist engineers engaged

upon such works_.”

=THE BUILDER=—_24th September, 1887_.

“A very practical little book, carefully compiled, and _one which all

writers of specifications for concrete work would do well to peruse_.”

“_The book contains reliable information for all engaged upon public

works._”

“A perusal of Mr. Newman’s valuable little handbook will point out the

importance of a more careful investigation of the subject than is

usually supposed to be necessary.”

AMERICAN PRESS.

=BUILDING=—_19th November, 1887_.

“To accomplish so much in so limited a space, the subject-matter has

been confined to chapters.”

“_We take pleasure in saying that this is the most admirable and

complete handbook on concretes for engineers of which we have

knowledge._”

LONDON: E. & F. N. SPON, 125, STRAND.

------------------------------------------------------------------------

TRANSCRIBER’S NOTES

1. Changed “the sinking a sump” to “the sinking of a sump” on p. 202.

2. Silently corrected typographical errors.

3. Retained anachronistic and non-standard spellings as printed.

4. Enclosed italics font in _underscores_.